A vibratory power screen normally has a generally planar and longitudinally extending frame having upstream and downstream ends and a pair of laterally spaced side members extending longitudinally between the ends. A generally planar screen is spanned longitudinally within the frame normally between its ends. The entire frame is tilted down from the upstream to the downstream end. Thus when particulate material is loaded onto the screen at the upstream end the particulate material migrates down along the screen to the downstream end with the smaller fractions falling through the screen.
The screen can be vibrated as described in commonly owned U.S. Pat. No. 4,319,993 wherein drive means is provided which includes a pair of drive motors fixed to the central member of the frame and respective eccentrics carried by the motors for limitedly displacing the frame and screen relative to the support. One of the motors is mounted on the high end below the plane of the screen and the other of the motors on the low end above the plane of the screen. In this case the frame is mounted, normally by suspension from stiff hangers, for limited displacement relative to a fixed support. Thus the entire frame and screen are shaken to sieve the material.
It is also known from German patent document No. 2,016,199 to provide the frame wih a plurality of longitudinally spaced and substantially parallel beater shafts underneath the screen. Respective beater arms extending generally parallel to the screen from the beater shafts have outer ends engageable with the underside of the screen. Respective motor/transmission drives are provided to oscillate the shafts and hammer the ends against the mesh, thereby vibrating the charge thereon. These multiple drive motors make the equipment quite expensive and hard to control.
Accordingly, in German patent document No. 2,133,187 a system is described wherein respective actuation arms extending radially from the beater shafts have outer ends all pivoted to a stiff longitudinally extending link. Drive means on the frame connected to the link simultaneously and synchronously pivotally oscillates the beater shafts with the respective beater arms to hammer the outer ends of the beater arms against the screen. In such an arrangement the frame need not be displaceable relative to a fixed support and a single drive can be used, invariably a crank arrangement on the rotary output of a motor/transmission unit. Although such an arrangement constitutes a considerable simplification and reduction in expensive parts, it substantially limits the adjustability of the system, as the stroke of the link is fixed and the frequency and force of the hammering on the bottom of the mesh is not readily controlled.
In copending application Ser. No. 406,955 filed Aug. 10 1982 (now U.S. Pat. No. 4,469,592) a system is described which is of the above-described general type, but which is provided with respective pivot plates each having one end rotatably supporting a respective end of one of the shafts and another end pivoted on a respective one of the side members at a respective adjustment axis for angular movement of said pivot plates with the respective shafts about the respective adjustment axes. The adjustment axes extend substantially parallel to one another transversely of the frame underneath the screen and are axially alignable with the outer ends of the respective actuation arms. Respective locking means can arrest pivotal movement of the pivot plates with the respective beater arms in any of a multiplicity of positions angularly offset relative to the alignment of the outer end of the respective actuating arm and the respective adjustment axis. With this system it is therefore possible to accurately adjust the positions of the beaters, for perfectly uniform beating action over the entire screen. This adjustment is, nonetheless, quite laborious as each beater must be positioned individually.
A common defect of all these systems is that the various pivot joints all require frequent servicing and attention. They are subject to considerable wear in the invariably dusty surrounding of the sieve, and therefore require periodic replacement even if properly maintained.